This invention relates to improvements in fabrication techniques for multilayer ceramic modules and, more particularly, to the metallization step of the through-holes punched in unfired ceramic sheets. It should be noted that the term "sheet" as used herein implies no limitation and that the method of the present invention also applies to automatized applications wherein unfired ceramic films or strips are continuously fed from a supply reel.
At the present time, unfired ceramic sheets are widely used in the microelectronics field and have played a significant part in the development of that industry. In one of the more common applications, the unfired ceramic sheet, after it has been subjected to suitable processing, is used as an insulating substrate onto which semiconductor chips may subsequently be deposited. It has been found that the ceramic materials involved exhibited all required properties, notably a high resistivity and a high mechanical strength.
Small size ceramic modules were initially used. These were provided on either or both of their main faces, using the silk-screening technique, with conductive metal patterns that served to electrically connect the active and/or passive components located on the module to the connection pins thereof and to external circuitry.
The significant advances subsequently made in the semiconductor industry to meet microminiaturization demands have made it possible to achieve high packing densities of components. For example, several hundreds, and possibly thousands, of transistors, diodes, resistors, etc., can be accommodated on a semiconductor chip having an area of the order of a few square millimeters. These advances have created a need for an insulating substrate such that the packing density of the conductive metal patterns referred to above would be very high.
The most promising solution to this problem is a well-known technique that relies upon the use of ceramics to provide multilayer circuit modules. These modules, the manufacturing steps of which are outlined hereafter, are multilayer ceramic structures each of which comprises a pattern of conductive metal lines and electric means for interconnecting the individual levels. These multilayer microelectronic ceramic structures are manufactured as follows: the first procedural step is to blend the raw components of the various ceramic bodies, in particulate form, and an organic vehicle, using, for example, a ball mill, to provide a viscous fluid commonly referred to as "paint." The latter is then deposited, utilizing a doctor blade or other well-known technique, onto a substrate to form a thin ceramic film adhering thereto. Generally, the doctor blade is stationary while the substrate is moving. The latter may consist, for instance, of a strip of plastic material such as polytetrafluorethylene (Teflon, DuPont de Nemours Reg. TM) or polyethylene tetraphtalate (Mylar, DuPont de Nemours Reg. TM), or of a web of stainless steel. The viscous fluid flows between the substrate and the blade to assume a constant, predetermined thickness. The resultant film is then dried at a drying station and stripped from its substrate. The thickness, porosity and other physical and electrical characteristics of the film are then checked. Additionally, the film is inspected to determine the presence of any cracks, blisters and other defects. Before it is used, the ceramic film is usually stored for some time in a suitable storage station to allow the remaining volatile components to evaporate. At this stage, the thin ceramic film is unfired, has a constant thickness, is free from the above-mentioned defects, and its external appearance is that of a plastified sheet. The film is then blanked into sheets of appropriate dimensions using standard techniques.
Through-holes are then punched at predetermined locations in the individual unfired ("green") sheets. A conductive paste is deposited by silk-screening or other equivalent technique on the surface of the sheets, to form the desired metallizations, and into the through-holes. This paste usually consists of powdered molybdenum, a glass frit and an organic vehicle essentially comprised of a binder and a solvent. The sheets are then aligned and stacked together in such a way that corresponding through-holes in different sheets are all located on the same vertical axis. The assembly is then laminated to provide good intersheet bonding through a softening of the resin binder. The lamination is perfomed at relatively low temperature and pressure. The laminated sheets are then sintered at a temperature that will allow the ceramic to densify, thereby eliminating the organic components and converting the conductive patterns formed as described above to the metal state. The resultant monolithic structure is then provided with connection pins or contact pads as well as with semiconductor chips.
One of the problems which arise during fabrication of multilayer ceramic structures is the metallization of the through-holes. These holes are punched in the green sheet using standard techniques and have a very small size, their diameter being of the order of 0.1 millimeter, so that they are extremely difficult to metallize. A standard process consists in mounting a silk-screening mask such as a molybdenum mask on the green ceramic sheet in which through-holes have been punched, the sheet itself being placed on a flat, stable carrier, and in forcing a metallization composition in particulate or paste form through the mask, using a doctor blade, so as to fill the holes and simultaneously form the desired metallization pattern on the surface of the green sheet. A similar method entitled "Vacuum Operated Silk Screening Technique" is described in IBM Technical Disclosure Bulletin, Vol. 16, No. 5, October 1973, page 1497. However, it would be desirable, instead of completely filling the holes as in the case of these prior art techniques, to achieve a partial metallization thereof, that is, to deposit a thin metallization layer on their inner surface, so as to leave a free space which could subsequently be filled by capillarity with a high conductivity metal such as copper, or, alternatively, accommodate a connection pin, as shall be seen hereafter.
Accordingly, it is an object of the present invention to overcome the disadvantages of the prior art and to provide a method of metallizing the through-holes punched in a green ceramic sheet, said method being characterized in that it enables a uniform, thin metallization layer to be deposited on the inner surface of each hole.
Another object of the invention is to provide green ceramic sheets comprising through-holes the inner surface of which is coated with a thin metallization layer and which can be filled by capillarity, after said sheets have been stacked together and laminated, with a metal such as copper.
Still another object of the invention is to provide a method of mounting connection pins into the through-holes of green ceramic sheets wherein the inner surface of said holes is coated with a thin metallization layer.
Yet another object of the invention is to provide a method of mounting connection pins into the through-holes punched in a green ceramic sheet and the inner surface of which is coated with a thin metallization layer, said method being characterized in that it can readily be automatized where said pins are cylindrical.
These and other objects are attained using the teachings of the present invention.